A lithium ion battery has been conventionally used as a power supply for a portable electronic apparatus. The lithium ion battery is generally configured by disposing a positive electrode, a negative electrode, and a separator in an electrolyte solution. In the positive electrode, lithium cobaltate or lithium manganate is applied to the surface of an aluminum foil. In the negative electrode, carbon is applied to the surface of a copper foil. The separator is disposed so as to separate the positive electrode and the negative electrode, to prevent a short circuit between the positive electrode and the negative electrode.
When the lithium ion battery is charged, lithium ions are released from the positive electrode and move to the negative electrode. In contrast, when the lithium ion battery is discharged, lithium ions are released from the negative electrode and move to the positive electrode. Such charge and discharge are repeated in the lithium ion battery. Therefore, it is necessary that the separator used in the lithium ion battery can allow lithium ions to well permeate there.
When the lithium ion battery is repeatedly charged and discharged, dendrites (dendrite crystals) of lithium are generated on the end surface of the negative electrode. The dendrites break through the separator to cause a very small short circuit (dendrite short circuit) between the positive electrode and the negative electrode, thereby remarkably deteriorating the battery capacity.
On the other hand, in order to improve the safety of the lithium ion battery, an olefinic resin porous film which mainly includes polyethylene is used as the separator. When the lithium ion battery generates abnormal heat due to a short circuit or the like, polyethylene constituting the porous film melts at about 130° C. to block a porous structure (shutdown function). In this manner, the abnormal heat of the lithium ion battery is stopped, and safety can be ensured.
In recent years, an increase in the output of a large-type battery such as a lithium ion battery for automobiles has proceeded. The temperature in the battery may be rapidly increased to higher than 130° C. For this reason, the shutdown function is not necessarily required for the separator for the lithium ion battery, and heat resistance is considered as an important factor. In order to increase the output of the lithium ion battery, a decrease in the resistance to permeation of lithium ions through the separator is required. Accordingly, it is necessary that the separator has high gas permeability. Further, it is important for the large-type lithium ion battery to ensure a long lifetime and long-term safety.
Various polypropylene porous films have been proposed as a separator having a high heat resistance. For example, Patent Literature 1 proposed a method for producing a polypropylene microporous film. The method includes extruding a composition containing polypropylene, a polymer having a melt crystallization temperature higher than that of polypropylene, and a β-crystal nucleating agent to mold the extruded composition into a sheet, and stretching the sheet at least uniaxially.
However, a polypropylene microporous film obtained by the method for producing a polypropylene microporous film has a low gas permeability and an insufficient lithium ion permeability. Therefore, it is difficult that such a polypropylene microporous film is used for the lithium ion battery requiring a high output.
Patent Literature 2 proposed a multilayered porous membrane which comprises a polyolefin resin porous membrane and a porous layer provided on at least one side of the polyolefin resin porous membrane, and has the degree of gas permeability of 1 to 650 sec./100 cc. The porous layer contains an inorganic filler or a resin having a melting point and/or a glass transition temperature of 180° C. or higher, and has a thickness of 0.2 μm to 100 μm. However, the multilayered porous membrane also has an insufficient lithium ion permeability, and therefore it is difficult that the multilayered porous membrane is used for the lithium ion battery requiring a high output.
In addition, Patent Literature 3 discloses a method for producing a polypropylene porous film, in which a polypropylene film is uniaxially stretched to form pores. However, in the polypropylene porous film obtained by the method of Patent Literature 3, pores are not uniformly formed. Therefore, the lithium ion permeability is not uniform. Accordingly, the polypropylene porous film includes a portion where the lithium ion permeability is high and a portion where the lithium ion permeability is low. Such a polypropylene porous film has disadvantages in which a very small short circuit is easily caused by the generation of dendrites in the portion where the lithium ion permeability is high and the long lifetime and the long-term safety are not sufficient.